Mechanisms and

Speciation

Populations evolve, not

individuals.

Natural selection acts on

distributions of traits. – A normal distribution

graphs as a bell-shaped curve.

– highest frequency near mean value

– frequencies decrease toward each extreme value

– Traits not undergoing natural selection have a normal distribution (no selective pressure).

Using your Data

Look at your Lima Bean Lab. Draw on

your graph a “bet fit bell curve.”

What was a limitation of the lab that

maybe decreased the chances of

seeing a bell curve?

Natural selection can change

the distribution of a trait in one

of three ways.

• Microevolution is evolution within a

population of one species.

– observable change in the allele

frequencies

– can result from natural selection

– Only affects one population of species

1-Directional selection favors phenotypes at

one extreme.

Example: Antibiotic drug resistance in

bacteria

Human Example: Lactose Tolerance • Before cattle domestication: tolerance mutation appeared

in only 1/100,000 individuals. Now, lactose tolerance (production of the lactase enzyme in adults) occurs in as much as 96% of the population in intense dairy cultures like Sweden, Denmark, parts of Australia, and Britain. Over 85% of European descent Americans are lactose tolerant too!

Think and Share

What is something in society in which

only one extreme is favorable?

–

2-Stabilizing selection favors the

intermediate phenotype . Example: mid-

size insect galls in plants caused by predator

selective pressure on both ends.

Galls are abnormal growths in plant

resulting from the work of immature

insects. In a way, they are basically "plant

tumors."

• https://braingenie.ck12.org/skills/104145/learn

Think and Share

What is something in society in which

both extremes are not favorable,

but the mean is favorable?

– 3-Disruptive selection favors

both extreme phenotypes.

Example: male bunting bird

plumage/wing coloration. Only

dominant brilliant colored

males and “dull” males are

fittest.

Think/Share

What is something in society in which

both extremes are favorable, but

the mean is not favorable?

What type of selection?

Selection type?

Selection type?

http://bcs.whfreeman.com/thelifewire/content/chp23/2302001.html

Concept Map

Natural selection is not the

only mechanism through

which populations evolve.

Gene flow is the movement of

alleles between populations caused

by migration of organisms. •Gene flow occurs when individuals join new populations and reproduce.

•Plant seeds/fungi spores are transported to new areas.

•Gene flow keeps neighboring populations similar.

•Low gene flow increases the chance that two populations will evolve into different species.

bald eagle migration

Two types of gene flow

migration: • Immigration (in-coming)– new

individuals come into a population,

with them they bring new genes.

• Emigration (exit) – individuals leave a

population, take genes with them and

can reduce the alleles in the

population.

Example:

Some individuals from a

population of brown

beetles might have joined

a population of green

beetles. That would make

the genes for brown

beetles more frequent in

the green beetle

population.

Genetic drift is a change in allele

frequencies due to chance. • Genetic drift causes a loss of genetic

diversity.

• It is most dramatic in small populations, but happens to all populations over time.

• Due to chance, some alleles may increase or decrease in frequency.

Example:

Imagine that in one generation, two brown beetles

happened to have four offspring survive to

reproduce. Several green beetles were killed

when someone stepped on them and had no

offspring. The next generation would have a few

more brown beetles than the previous generation

— but just by chance. These chance changes from

generation to generation are known as genetic

drift.

Example:

Remember Lactose Intolerance? Sometimes

natural selection and genetic drift combine to

increase the frequency of traits in a population.

Many scientists think that some African

populations became very small at the same time

that the lactose tolerance gene appeared.

• Genetic drift can cause big losses of

genetic variation for small

populations.

• Two types of genetic drift:

– Bottleneck

– Founder Effect

Bottleneck Effect Population bottlenecks occur when a

population’s size is reduced for at

least one generation.

• Reduces genetic variation.

Bottleneck Example • Cheetahs: Cheetahs

are sufficiently closely related to one another that transplanted skin grafts do not provoke immune responses, thus suggesting an extreme population bottleneck in the past.

• The founder effect is genetic drift that

occurs after start of new population. It

occurs when a few individuals start a new

population (like colonists).

• 10%-80% of seeds available at night are taken by bats.

• Bats and other mammals are effective dispersers of

cactus seeds, depositing them under the tree canopies

(Von Rheede Von Oudtshoorn 1999)

Genetic drift has negative

effects on a population.

Why would more

variation be better for a

population?

Genetic drift has negative

effects on a population.

– Less genetic variation for a population

is bad.

– Less likely to have some individuals

that survive so the population can

adapt

– Harmful alleles can become more

common due to chance

New species can arise when

populations are isolated

First, define a species:

• A group of individuals that can

interbreed to produce fertile

offspring.

The isolation of populations

can lead to speciation. • Speciation is the rise of two or more species from

one existing species.

• Populations become isolated when there is no gene

flow.

• However, isolation, by itself doesn’t necessarily

mean that speciation will occur.

There are two main causes of speciation:

A. Geographic Isolation and

B. Reproductive isolation

Geographic isolation

A. Physical barriers divide population

Reproductive isolation

B. Members of different populations cannot

mate successfully. This increases the

chance of speciation-creation of a new

species!

Reproductive Isolation

• Isolated populations adapt to their own

environments in specialized ways (niches).

• Genetic differences can add up over generations.

Reproductive isolation

There are three types of

reproductive isolation:

1-Temporal isolation: Timing of

reproductive periods prevents

mating (examples: nocturnal vs

diurnal or seasonal differences)

Reproductive Isolation

2. Behavioral isolation - includes

differences in courtship or mating

behaviors that prevents reproduction

between the two populations

3. Hybrid infertility Even if fertilization occurs

successfully, the offspring may not

survive, or if it survives may not

reproduce.

https://www.youtube.com/watch?v=JE8Z

5Es-M0E

Species can become extinct.

• Extinction is the elimination of a species from Earth.

• Background extinctions occur continuously at a very low rate.

– occur at roughly the same rate as speciation

– usually affects a few species in a small area

– caused by local changes in environment

Mass extinctions are rare but much more intense.

– destroy many species at global level

– thought to be caused by catastrophic events

– at least five mass extinctions in last 600 million years (1) Ordovician-Silurian mass extinction-all sea life 2) Devonian-sea life extinction 3) Permian extinction 96% species extinct 4) Triassic and 5)Cretacious

dino

Tropical Island

• Climate: This island has a warm average temperature of 80 degrees. Temperature only fluctuates a maximum of 20 degrees +/- on a daily and annual basis. The days are marked with intermittent heavy rain storms, which contribute to this islands lush foliage.

• Food: The most abundant foods are fish and fruits. The fish are all found in the nearby ocean, and the fruit are found at the tops of very tall trees.

• Predators: There are jungle cats in this area. Also if you eat the wrong type of frog or fruit they could be poisonous.

Desert Island • Climate: This island is a dry, dusty island marked with extremely

hot days up to 120 degrees and extremely cold nights to near freezing. There is little rain fall and there are extreme wind storms that create severe dust clouds.

• Food: There are several different species of snakes and rabbits that can be found on the island, however they are difficult to find during periods of extreme heat. Fresh water is found only sporadically, so it must be conserved. The only vegetation available is cactus.

• Predators: There are coyotes in this area as well as cougars.

Arctic Island

• Climate: This island is covered in snow for 10 months out of the year. Winters months are very cold with temperatures rarely going above 10 degrees and only 2-4 hours of daylight. Summer months are marked by almost 20 hours of daylight and temperatures in the 50’s.

• Food: There is a small population of arctic foxes and penguin-like birds. However, it is difficult to hunt them during the winter months. Ice-fishing allows for most of the food supply. Vegetation can only be found during the summer months.Predators: Wolves

Coniferous Island

• Climate: This is a very mountainous terrain, covered in pine trees. The area is covered in snow for 1/3 of the year. There is a rainy season for 3-4 months and a summer season with highs in the 70’s.

• Food: There are many birds that live in the trees, fish in the fresh water river and lakes, and a population of large bear that compete for fish.

• Predators: Large cats, bears and wolves.

Grassland

• Climate: The average temperature is warm (80 degrees), with summer days reaching 110 and winter days reaching 60 degrees. There is moderate rain fall to provide water for many small plants and scattered trees.

• Food: This area has a population of antelope that feed on the grass and a population of mountain lion that feed on the antelope and sometimes people. There are many insects that live in the grasses.

• Predators: Lions, hyenas, and killer bees.

Water World

• Climate: The climate is very stable, with warm comfortable temperatures all the time in the air, but there are no land masses available for habitation. The water a comfortable temperature, but there are occasionally storms in which there are turbulent waves and currents.

• Food: Many varieties of fish if you can swim fast enough to catch them. Crabs and lobster are on the floor of the water, and there is edible seaweed that floats in “forests” throughout the large body of water.

• Predators: Sharks